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President Barack Obama holds a narrow advantage in the contest for the electoral votes needed to win the White House, though he and challenger Mitt Romney remain in a virtual tie in the popular vote.

In Congress, despite record levels of disapproval with the institution, voters seem likely to opt for the status quo — Democrats in charge of the Senate and Republicans in the House.

Democrats are expected to gain seats in the House but not the 25 needed to recapture the majority lost in the Republican sweep of 2010. In the Senate, Democrats hold a 53-47 majority, including two independents who caucus with them. Although 10 or more races were considered too close to call through much of the fall, Democrats are now in a position to maintain their majority, although perhaps barely.

This election assessment, along with reports on all the states, is based on interviews by a team of Washington Post reporters with strategists in the two parties and both presidential campaigns, as well as state and local officials and independent analysts. The assessment includes an analysis of polls on individual states and races that have poured forth over the final weeks before the election.

In the presidential campaign, the biggest and most consequential unknowns at this point are the size and shape of the voting population. An electorate that resembles or even slightly exceeds 2008 in terms of the share of minority voters vs. white voters would clearly benefit Obama. A slight decline in the minority share of the electorate and a more even split between Democrats and Republicans — closer to the 2004 electorate than 2008 — would greatly help Romney. Obama, however, may do better among white voters in some of the battleground states than he will do nationally.

The latest Washington Post-ABC News national poll shows a dead heat this weekend, with Obama and Romney both at 48 percent among likely voters. The survey, which began after the final presidential debate, has barely fluctuated.

Maine, self-inflicted wounds in Senate

Democrats appear poised to hold on to their narrow Senate majority on Tuesday, a prospect that as recently as a year ago seemed far more difficult, given the disparity in the number of seats Democrats (23) and Republicans (10) had to defend.

The turning point for Democrats may have been the surprise decision by Republican Sen. Olympia J. Snowe (Maine) not to seek another term. Snowe was considered a shoo-in for reelection but in her absence, former governor Angus King, an independent who would caucus with Democrats, has emerged as a favorite.

Then Republicans made a series of self-inflicted mistakes, two centering on the subject of rape. Indiana Treasurer Richard Mourdock, who ousted longtime Sen. Richard G. Lugar in the GOP primary, gave Democratic Rep. Joe Donnelly an opening. Then Mourdock’s comments at a late October debate — during which he said a woman who becomes pregnant by rape is carrying a “gift from God” and therefore must have the child — turned the race from a tossup to one that favors Donnelly.

In Missouri, Rep. Todd Akin won the GOP nomination and days later made a comment about “legitimate rape” rarely causing pregnancy that created a national firestorm. Much of the party establishment abandoned Akin, but he refused to drop out of the race. Sen. Claire McCaskill, a Democrat, is favored to win reelection.

Democrats have growing confidence that former Obama administration official Elizabeth Warren, a Harvard law professor, will beat Republican Sen. Scott Brown in Massachusetts. The contest is the premier Senate race in the country for the seat long held by the late Edward M. Kennedy, a Democrat.

Republicans have a number of opportunities to pick up seats held by retiring Democrats. Races in Nebraska and North Dakota, where Democrats are retiring, seem likely to fall for Republicans. Virginia has a very tight race between two former governors: Democrat Timothy M. Kaine and Republican George Allen. In Montana, Sen. Jon Tester, a Democrat, and Rep. Denny Rehberg, a Republican, have been tied for months.

Even if several of those races tip to Republicans in the final hours, it still seems unlikely that the GOP will add the four seats it would need to control the Senate if Obama is reelected or the three it would would need if Romney wins. The most likely outcome is no net change, or Republican gains of one or two seats.

In the battle for control of the House, it has become clear over the past few months that the wave elections that roiled the chamber in 2006, 2008 and 2010 will not be repeated Tuesday. Democrats need 25 seats to regain the majority. Not even their most optimistic strategists think that is anything more than a remote possibility.

Because of the decennial redistricting process, both parties will score heavy gains in certain states. Republicans look poised to pick up at least three and maybe four seats in North Carolina. Democrats could take five seats from Republicans in Illinois.

After taking control of more governorships and state legislatures in 2010, Republicans had the political muscle to carve up new congressional districts to their advantage. They shored up some of their most vulnerable incumbents and drew difficult districts for Democrats.

The result: Republicans are now likely to gain at least 10 Democratic seats simply because of the partisanship of redrawn districts, and Democrats are fighting from behind in their effort to win back the House. That is why overall Democratic gains may be held to the mid-single digits.

Obama spent part of this past week in his official capacity as commander in chief as Hurricane Sandy devastated the Atlantic Coast, pummeling New Jersey and New York the hardest. His attention to the cleanup earned praise from New Jersey Gov. Chris Christie, a Romney supporter who delivered the keynote address at the Republican National Convention.

Almost half of all Americans said Obama’s hurricane response would be a factor in their vote, according to the latest Post-ABC tracking poll. An earlier survey found that 79 percent rated his handling of the situation excellent or good.

Another wild card is whether the latest jobs report will have a demonstrable effect on an electorate deeply polarized and with few undecided voters left. The report, released Friday, showed that 171,000 jobs were added while the unemployment rate ticked up to 7.9 percent.

Through much of the fall campaign, nine states have defined the presidential battleground: Colorado, Florida, Iowa, Nevada, New Hampshire, North Carolina, Ohio, Virginia and Wisconsin.

Obama began the general-election race with a base of 18 states plus the District of Columbia, totaling 237 electoral votes. Romney began with a base of 23 states, totaling 191 electoral votes. North Carolina is tipping toward Romney and Nevada toward Obama, putting the president at 243 and Romney at 206.

Romney is making a late play in Pennsylvania and Minnesota and will campaign in Pennsylvania on Sunday. Both states continue to lean toward the president, but Obama’s campaign has decided to send former president Bill Clinton to Pennsylvania on Sunday for extra measure.

Assuming those states continue to stay in Obama’s column, the president would need only 27 of the remaining 89 electoral votes to win. Romney would need 64 of the 89, which explains why Obama still has an easier — but by no means certain — path to an Electoral College majority. For example, he could win a second term simply by winning Florida, which remains competitive.

If Florida goes for Romney, then much will depend on Ohio, which is why it is the focus of so much campaign activity in the final days. Its 18 electoral votes represent the bulwark of Obama’s Midwestern line of defense against Romney.

If the president were to carry Ohio — he continues to hold a narrow lead in public polls there — he could win an electoral majority by adding Virginia (13 electoral votes) or Wisconsin (10) or Colorado (nine), or by winning Iowa (six) and New Hampshire (four).

If Romney does not win Ohio, his path to victory would have to include Colorado, Florida, Virginia, Wisconsin and either Iowa or New Hampshire. But if he does capture Ohio, he could become president by taking Florida and Virginia and then just one other contested state.

The Post-ABC tracking survey underscored the closeness of the race nationally. During two weeks of polling, the largest lead by either candidate was 50 percent to 47 percent, favoring Romney. Obama’s biggest was one point. Neither was statistically significant.

Obama’s job approval rating continues to hold at 50 percent, with 34 percent saying they strongly approve and 39 percent saying they strongly disapprove. Romney holds a statistically insignificant three-point edge on who is trusted more to handle the economy. Obama has a six-point advantage on who better understands Americans’ economic problems.

Through most of the campaign and as recently as October, more voters offered unfavorable than favorable impressions of Romney. Today a majority of likely voters has a favorable view. In fact, he and Obama are virtually tied on this measure: 54 percent say they have a favorable impression of the president, while 53 percent say the same of Romney.

For the first time in the Post-ABC poll, independent voters are evenly split between the two candidates, at 46 percent each. Until now, Romney has held an advantage ranging from three to 20 points. Obama leads among women by six points, Romney among men by seven points.

Obama is winning 38 percent of white voters and 78 percent of non-whites. He gets 33 percent of whites without college degrees and 44 percent of whites with college degrees.

Obama’s hopes for a second term rest on his ability to reassemble and motivate a coalition of African Americans, Hispanics, young voters and women, despite disappointment and diminished enthusiasm since his historic 2008 victory.

In Ohio, he is aided by the success of the auto-industry bailout and the campaign’s attacks on Romney’s business background, which have bolstered the president’s support among white working-class voters. Beyond that, Romney has been hammered in Ohio for an ad suggesting that Chrysler’s new owners plan to shift production of Jeeps to China, which the company’s chief executive denied.

Romney has a motivated base, with Republicans eager to defeat a president they think is taking the country in the wrong direction. But he needs a turnout that is large enough to make the GOP share of the electorate almost equal to that of the Democrats. He also needs an edge among independent voters.

For the past week, Obama’s advisers have expressed confidence that the race is theirs to lose and that they will not lose it. “The economic debate has crystallized,” campaign manager Jim Messina said Saturday. “We have picked up steam and now what we have to do is turn out our vote. We continue to lead or are tied in every battleground state and have the ability to get to 270 electoral votes in a variety of ways.”

Romney advisers have said their own polls show that the battlegrounds, particularly Ohio, are closer than public surveys suggest. They also say that an incumbent who is not above 50 percent in the polls in the final days before an election is in a precarious position.

“When you take a look at the big three — Florida, Virginia and Ohio — we feel very good,” said Romney senior adviser Russ Schriefer. “Ohio is tight but it’s tied and I think we’ve got some advantages there. I think then you look at the rest of the map that, in a million years, the Obama campaign never thought they would be campaigning in these states the weekend before the election.”